Polyolefins tend to have excellent physical and chemical properties. Improvement of polymer properties is a dominant factor in the development and production of olefin polymers. Several methods have been employed to improve various polymer properties. The prior art teaches that reinforcing agents, such as glass, can be incorporated into the polymer to improve the mechanical properties and/or the heat resistance of the polymer. However, these improved properties cannot be attained by merely mixing the glass fibers and the polyolefins together because the bonding strength between the glass fibers and the polyolefin is weak. Thus, such polymers must have a more "bondable" component grafted thereon to facilitate reinforcement with glass fibers and other generally infusible reinforcing agents.
Polymers with relatively high melting points, such as stereoregular polymers of branched, higher alpha-olefins, have been developed. These polymers are useful in high temperature applications, such as microwave packaging. Improving the performance and/or properties of these polymers could expand the uses of these polymers. Because of the relatively high melting points and even higher melt processing temperatures, these polymers can be thermally unstable. Therefore, it is difficult to graft, and optionally reinforce, stereoregular polymers of branched, higher alpha-olefins in a melt process, as can be done with other polyolefins such as polyethylene or polypropylene.
Grafting, or chemically modifying, polyolefins, especially relatively low melting point polymers, such as polyethylene and polypropylene, can be done alone or in combination with the addition of a reinforcing agent. The prior art teaches many grafting compounds, as well as several grafting processes. Grafting usually is done in the presence of a free radical generator and usually by a solution or melt process. Unfortunately, the presence of a free radical generator is also known to cause degradation of the polymer. Furthermore, a melt grafting process is also known to be detrimental to the properties of stereoregular polymers of branched, higher alpha-olefins.
Polyolefins are vulnerable to thermal-oxidative degradation due to exposure to heat or oxygen during processing and/or storage. To preserve desirable polymer properties, an antioxidant or stabilizer is sometimes used to inhibit discoloration and/or molecular deterioration. An unstabilized polyolefin can be susceptible to rapid deterioration over time. Unfortunately, the use of a stabilizer can have a detrimental effect on the grafting process because stabilizers function as radical scavengers and peroxide decomposers and consequently stabilizers can hinder, if not halt, the grafting process.
Recently, a new class of aliphatic thio compounds has been introduced to the market. See Novel Sulfide Antioxidant Stabilizes Polyolefins and ABS, Vol. 34, No. 4, Plastics Technology, p. 28 (April 1988). This new class of aliphatic thio compounds has been disclosed as giving superior high performance properties when used with polyolefins. See Braksmayer Antioxidant synergist cuts odor, protects color, Vol. 66, No. 1, Modern Plastics, p. 122 (January 1989). It was disclosed recently that this class of aliphatic thio compounds, in particular Anoxsyn.TM. 442, did not interfere with grafting reactions. See Callais and Kazmierczak The Maleic Anhydride Grafting of Polypropylene with Organic Peroxides, ANTEC '90, p. 1921 (1990). It was soon discovered that the Anoxsyn.TM. 442 while imparting certain beneficial properties, also produced a significant decline in the mechanical properties of the polyolefin composition it was in. Consequently, before the beneficial properties of the aliphatic thio compounds could be used, the antagonistic interaction between the grafting reaction and the aliphatic thio compounds must be reduced.